1. Field of the Invention
This invention relates to washers for high tensile strength nuts and bolts and particularly to washers having means for indicating when a predetermined tension has been realized on the bolt.
2. The Prior Art
The amount of tension in a high tensile strength bolt is a critical parameter in steel fabrication. If the bolt is not tight enough, then the joint of which the bolt is part will not be as strong as designed. On the other hand, if the bolt is overstressed, a potential future failure is built into the joint.
Among the most common techniques for determining the tension in a bolt is by the use of a wrench capable of delivering a certain torque, which torque is equated to a predetermined tension on the bolt. This common technique is recognized by everyone skilled in the art to be one that is highly variable in result due to significant variations in friction which could lead to requirements for high or low torque to twist a bolt at any predetermined actual tension. Thus, the correlation is rough and the indication can prove highly inaccurate. Such a method is described and its disadvantages are discussed in an article which appeared in the Mar. 6, 1975 issue of Machine Design at pages 78 to 82.
The deficiencies in a method of determining tension by measuring torque have been recognized for a very long time and there have been numerous attempts to provide ways of yielding a more direct technique for accomplishing this desirable end. Thus, in Seitz U.S. Pat. No. 2,394,812 granted on Feb. 12, 1946, it is proposed to provide such an indicating means by employing two nuts which are soldered together and then applying a wrench to one of the nuts, specifically the outer one, and tightening the two nuts until the torque on the nut actually engaged by the wrench becomes so great relative to the other nut that the solder is sheared. When the outer nut is thus severed from the inner, an indication of the bolt tension is given. The Seitz method has several aspects that have limited commercial realization. First, it requires two nuts which add to the cost. Secondly, the Seitz lock nut still relies on torque as an indication of bolt tension since it is the shear force resulting from torquing that will cause the shearing of the solder between the two nuts. This being so, the Seitz method is still an indirect method with all of the resulting shortcomings of the standard torque method.
In Lewis U.S. Pat. No. 3,174,386, a special washer is disclosed which yields a direct indication of bolt tension. This is accomplished by providing the washer with two flared out portions and a necked down center, which necked down center will actually collapse when the center in the bolt reaches a predetermined level. While Lewis thus gives a direct indication of bolt tension, rather than an indirect indication determined from torque, the Lewis washer tends to be costly to manufacture and the indicating portion is recessed between the bolt head and the adjacent plate and is not readily visible to the person applying bolt tension. Moreover, it requires soft steel which is contrary to acceptable modern practice of forming washers for high strength bolts from heat treated hardened steel.
It has been proposed to incorporate in bolt heads or in washers a variety of materials which change their optical properties, such as color or plane of polarization, when subjected to certain minimum amounts of tension. For example, reference may be had to the Grikscheit et al U.S. Pat. Nos. 3,224,316 and 3,495,907. However, to the best of applicant's knowledge, such optical indicating means have not achieved any degree of commercial success, partly due to the inclusion of soft optical materials therein.
In Velthoven U.S. Pat. No. 3,783,734, it is proposed to use a specially designed washer in conjunction with a specially designed nut or bolt head which cooperates to cause a frangible piece of the specially designed washer to break off when a predetermined tension is achieved in the bolt. The specially designed nut or bolt head, however, requires a reduction in the bearing surface provided by said nut or bolt head, thereby to reducing the effective tension capable of being handled by said nut or bolt.
In Crites U.S. Pat. No. 3,788,186, a stress indicating means is described which provides for the incorporation of a frangible insert in the head of a bolt or on a washer, which frangible insert will break when a tension of certain proportions is achieved in the bolt. In the usual tightening of such a bolt by a socket wrench, the socket covers and therefore conceals the indicating element during the course of tightening. Also, the Belleville type washer is not allowed by most Codes for use with high tensile bolts.
There are a number of commercially available washers presently on the market which do yield direct indications of bolt tension. Thus, for example, there is a four-piece washer assembly termed "preload indicating washer" that is manufactured by The Standard Pressed Steel Company which is described in a publication entitled Fasteners Handbook at page 278. This same washer assembly is described in the aforementioned article in Machine Design at page 81. In addition, the Illinois Tool Works, Inc. markets a so-called "square cone washer" which is apparently described in U.S. Pat. No. 3,628,584. Additionally, Cooper & Turner, Inc. and Bethlehem Steel Company both market a load indicator washer each of which is apparently described in U.S. Pat. No. 3,187,621 which is owned by Cooper & Turner, Inc. However, all three of these commercially available washers yield an indication that is not readily visible and often requires a gauge or the like in order to determine precisely what is being indicated by them. The "square cone washer" is essentially a Belleville type washer with the same objections cited for Crites. The Bethlehem washer does not provide full bearing between the undersurface of the nut or bolt and the top of the washer. In high heat applications this could lead to a relaxation of bolt tension.